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Text of the page (random words):
success is the single shot device or system these are devices or systems that remain relatively dormant and only operate once examples include automobile airbags thermal batteries and missiles single shot reliability is specified as a probability of one time success or is subsumed into a related parameter single shot missile reliability may be specified as a requirement for the probability of a hit for such systems the probability of failure on demand pfd is the reliability measure this is actually an unavailability number the pfd is derived from failure rate a frequency of occurrence and mission time for non repairable systems for repairable systems it is obtained from failure rate mean time to repair mttr and test interval this measure may not be unique for a given system as this measure depends on the kind of demand in addition to system level requirements reliability requirements may be specified for critical subsystems in most cases reliability parameters are specified with appropriate statistical confidence intervals reliability testing edit the purpose of reliability testing or reliability verification is to discover potential problems with the design as early as possible and ultimately provide confidence that the system meets its reliability requirements the reliability of the product in all environments such as expected use transportation or storage during the specified lifespan should be considered 10 it is to expose the product to natural or artificial environmental conditions to undergo its action to evaluate the performance of the product under the environmental conditions of actual use transportation and storage and to analyze and study the degree of influence of environmental factors and their mechanism of action 24 through the use of various environmental test equipment to simulate the high temperature low temperature and high humidity and temperature changes in the climate environment to accelerate the reaction of the product in the use environment to verify whether it reaches the expected quality in r d design and manufacturing 25 reliability verification is also called reliability testing which refers to the use of modeling statistics and other methods to evaluate the reliability of the product based on the product s life span and expected performance 26 most product on the market requires reliability testing such as automotive integrated circuit heavy machinery used to mine nature resources aircraft auto software 27 28 reliability testing may be performed at several levels and there are different types of testing complex systems may be tested at component circuit board unit assembly subsystem and system levels 29 the test level nomenclature varies among applications for example performing environmental stress screening tests at lower levels such as piece parts or small assemblies catches problems before they cause failures at higher levels testing proceeds during each level of integration through full up system testing developmental testing and operational testing thereby reducing program risk however testing does not mitigate unreliability risk with each test both statistical type i and type ii errors could be made depending on sample size test time assumptions and the needed discrimination ratio there is risk of incorrectly rejecting a good design type i error and the risk of incorrectly accepting a bad design type ii error it is not always feasible to test all system requirements some systems are prohibitively expensive to test some failure modes may take years to observe some complex interactions result in a huge number of possible test cases and some tests require the use of limited test ranges or other resources in such cases different approaches to testing can be used such as highly accelerated life testing design of experiments and simulations the desired level of statistical confidence also plays a role in reliability testing statistical confidence is increased by increasing either the test time or the number of items tested reliability test plans are designed to achieve the specified reliability at the specified confidence level with the minimum number of test units and test time different test plans result in different levels of risk to the producer and consumer the desired reliability statistical confidence and risk levels for each side influence the ultimate test plan the customer and developer should agree in advance on how reliability requirements will be tested a key aspect of reliability testing is to define failure although this may seem obvious there are many situations where it is not clear whether a failure is really the fault of the system variations in test conditions operator differences weather and unexpected situations create differences between the customer and the system developer one strategy to address this issue is to use a scoring conference process a scoring conference includes representatives from the customer the developer the test organization the reliability organization and sometimes independent observers the scoring conference process is defined in the statement of work each test case is considered by the group and scored as a success or failure this scoring is the official result used by the reliability engineer as part of the requirements phase the reliability engineer develops a test strategy with the customer the test strategy makes trade offs between the needs of the reliability organization which wants as much data as possible and constraints such as cost schedule and available resources test plans and procedures are developed for each reliability test and results are documented reliability testing is common in the photonics industry examples of reliability tests of lasers are life test and burn in these tests consist of the highly accelerated aging under controlled conditions of a group of lasers the data collected from these life tests are used to predict laser life expectancy under the intended operating characteristics 30 reliability test requirements edit there are many criteria to test depends on the product or process that are testing on and mainly there are five components that are most common 31 32 product life span intended function operating condition probability of performance user exceptions 33 the product life span can be split into four different for analysis useful life is the estimated economic life of the product which is defined as the time can be used before the cost of repair do not justify the continue use to the product warranty life is the product should perform the function within the specified time period design life is where during the design of the product designer take into consideration on the life time of competitive product and customer desire and ensure that the product do not result in customer dissatisfaction 34 35 reliability test requirements can follow from any analysis for which the first estimate of failure probability failure mode or effect needs to be justified evidence can be generated with some level of confidence by testing with software based systems the probability is a mix of software and hardware based failures testing reliability requirements is problematic for several reasons a single test is in most cases insufficient to generate enough statistical data multiple tests or long duration tests are usually very expensive some tests are simply impractical and environmental conditions can be hard to predict over a systems life cycle reliability engineering is used to design a realistic and affordable test program that provides empirical evidence that the system meets its reliability requirements statistical confidence levels are used to address some of these concerns a certain parameter is expressed along with a corresponding confidence level for example an mtbf of 1000 hours at 90 confidence level from this specification the reliability engineer can for example design a test with explicit criteria for the number of hours and number of failures until the requirement is met or failed different sorts of tests are possible the combination of required reliability level and required confidence level greatly affects the development cost and the risk to both the customer and producer care is needed to select the best combination of requirements e g cost effectiveness reliability testing may be performed at various levels such as component subsystem and system also many factors must be addressed during testing and operation such as extreme temperature and humidity shock vibration or other environmental factors like loss of signal cooling or power or other catastrophes such as fire floods excessive heat physical or security violations or other myriad forms of damage or degradation for systems that must last many years accelerated life tests may be needed testing method edit a systematic approach to reliability testing is to first determine reliability goal then do tests that are linked to performance and determine the reliability of the product 36 a reliability verification test in modern industries should clearly determine how they relate to the product s overall reliability performance and how individual tests impact the warranty cost and customer satisfaction 37 accelerated testing edit main article accelerated life testing the purpose of accelerated life testing alt test is to induce field failure in the laboratory at a much faster rate by providing a harsher but nonetheless representative environment in such a test the product is expected to fail in the lab just as it would have failed in the field but in much less time the main objective of an accelerated test is either of the following to discover failure modes to predict the normal field life from the high stress lab life an accelerated testing program can be broken down into the following steps define objective and scope of the test collect required information about the product identify the stress es determine level of stress es conduct the accelerated test and analyze the collected data common ways to determine a life stress relationship are arrhenius model eyring model inverse power law model temperature humidity model temperature non thermal model software reliability edit further information software quality and site reliability engineering software reliability is a special aspect of reliability engineering it focuses on foundations and techniques to make software more reliable i e resilient to faults system reliability by definition includes all parts of the system including hardware software supporting infrastructure including critical external interfaces operators and procedures traditionally reliability engineering focuses on critical hardware parts of the system since the widespread use of digital integrated circuit technology software has become an increasingly critical part of most electronics and hence nearly all present day systems therefore software reliability has gained prominence within the field of system reliability there are significant differences however in how software and hardware behave most hardware unreliability is the result of a component or material failure that results in the system not performing its intended function repairing or replacing the hardware component restores the system to its original operating state however software does not fail in the same sense that hardware fails instead software unreliability is the result of unanticipated results of software operations even relatively small software programs can have astronomically large combinations of inputs and states that are infeasible to exhaustively test restoring software to its original state only works until the same combination of inputs and states results in the same unintended result software reliability engineering must take this into account despite this difference in the source of failure between software and hardware several software reliability models based on statistics have been proposed to quantify what we experience with software the longer software is run the higher the probability that it will eventually be used in an untested manner and exhibit a latent defect that results in a failure shooman 1987 musa 2005 denney 2005 as with hardware software reliability depends on good requirements design and implementation software reliability engineering relies heavily on a disciplined software engineering process to anticipate and design against unintended consequences there is more overlap between software quality engineering and software reliability engineering than between hardware quality and reliability a good software development plan is a key aspect of the software reliability program the software development plan describes the design and coding standards peer reviews unit tests configuration management software metrics and software models to be used during software development in modern distributed and microservices based systems reliability considerations increasingly extend beyond component level correctness to include the behavior of applications at the business logic level while traditional software reliability engineering and site reliability engineering sre focus on system level indicators such as availability and latency some sources and industry practitioners refer to this application focused perspective as application reliability engineering are this perspective emphasizes end to end user transactions and the correctness of business workflows including metrics such as transaction success rates data consistency and workflow completion across multiple interacting services it addresses scenarios in which individual system components may appear operational while failures arise from service interactions or logic errors potentially leading to degraded user experience or incorrect outcomes practices associated with application level reliability include end to end observability synthetic transaction testing and validation of business critical processes complementing traditional infrastructure focused reliability methods a common reliability metric is the number of software faults per line of code floc usually expressed as faults per thousand lines of code this metric along with software execution time is key to most software reliability models and estimates the theory is that the software reliability increases as the number of faults or fault density decreases establishing a direct connection between fault density and mean time between failure is difficult however because of the way software faults are distributed in the code their severity and the probability of the combination of inputs necessary to encounter the fault nevertheless fault density serves as a useful indicator for the reliability engineer other software metrics such as complexity are also used this metric remains controversial since changes in software development and verification practices can have dramatic impact on overall defect rates software testing is an important aspec...
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